FIG. 1 shows an O-frame thickness measurement gauge sensor scanner 10. O-frame thickness measurement gauge sensor scanners are commonly used to measure the thickness and/or basis weight of metal, plastic, rubber, nonwoven or other web materials 20 produced in sheet or strip form. The web material 20 is passed through the O-frame opening 12 of a defined width 14 and height 16 and between sensors 22, 24 disposed on opposing faces of the frame 12. In many cases, the sensors are non-contact sensors. Drawbacks of O-frame thickness measurement gauge scanners include that the fixed distance and registration of sensor pairs are difficult to achieve and maintain stably. Weaknesses are in part directly linked to inherent registration issues of the independent mounting surfaces provided by the translating elements incorporated in the O-frame. Moreover, as the width of a web widens, there is greater difficulty to maintain registration and distance of supporting structures of the sensor components, which adversely impacts the quality of the measurements. Furthermore, mechanical wear and tear on translating elements can lead to long term degradation of registration, which directly affects the integrity and reliability of the measurement over time.
C-frame scanning gauges with non-contact sensors sometimes are used to measure thickness and/or basis weight of a web material. In many cases, use is limited to measuring along either or both edges of the web owing to drawbacks in large through depth geometries. Once such drawback is best understood by viewing the arms of the C frame construction as a pair of cantilevered beams. As beam length increases, the tips of the beams to which sensor pairs are affixed are subject to ever greater upsets in positional stability, both individually and differentially. A second drawback can be seen by the negative impact of off line excursion requirements. In the off-line position, the rear locus of the C Frame often violates the spatial constraints of a given process envelope as it relates to available aisle space and/or prohibitive impingement on regions directly adjacent to the process apparatus. These off-line excursions are frequently mission critical (a) to avoid mechanical interference of the sensors with the web during string-up operations and/or (b) to provide periodic retraction of the sensors from the running web for dynamic re-standardization purposes.
The industry continues to seek improvements and effective alternatives for apparatus and methods to determine accurate thickness of web and film materials.